Penelope Fillon was paid a salary from public funds allocated to François Fillon as an MP for the Sarthe region.
Photograph: Lionel Bonaventure/AFP/Getty

The rightwing French presidential candidate François Fillon is under pressure to explain the role of his British wife in his political operation after a newspaper alleged that she had been paid about €500,000 (£430,000) in eight years out of parliamentary funds.

The satirical and investigative weekly Le Canard Enchaîné claimed that there were various periods during which Penelope Fillon, who was born in Wales, was paid a generous salary from public funds that were allocated to her husband as an MP for the central Sarthe region to pay for parliamentary staff.

Hiring family members is legal for French MPs and not against parliamentary rules as long as the person is genuinely employed. But the newspaper claimed it had been unable to track down anyone who had seen evidence of Penelope Fillon’s work. Until now she has been regarded as not having played a major role in her husband’s political life.

On a visit to Bordeaux, Fillon told reporters that he was “scandalised” by the Canard Enchaîné article, which he described as “misogynistic”. He said: “So, because she’s my wife she shouldn’t be allowed to work?” He called it campaign mud-slinging but did not reply in detail about the nature of his wife’s work for him.

Fillon is under pressure to provide swift proof of the parliamentary assistant work carried out by his wife who, although she was elected to a village council in 2014 in Sarthe and has been present recently on the election campaign trail, has generally presented herself as not playing a frontline role in her husband’s political activities.

The financial prosecutors’ office in Paris said in a statement on Wednesday it had opened an inquiry into the misuse of public funds.

The issue is all the more pressing for Fillon because, despite 35 years in politics including five years as prime minister, he is styling himself as an anti-system candidate, promoting himself as an honest, austere and “irreproachable” antidote to years of corruption scandals on the French right.

He is currently considered a frontrunner to make it through to the final round of the highly unpredictable French presidential election in May, alongside the far-right Front National’s Marine Le Pen. He also faces a challenge from the former economy minister Emmanuel Macron, who is running on a maverick outsider ticket as “neither left nor right”.

The former prime minister Manuel Valls, who is running for the Socialist party’s nomination as presidential candidate, told France Inter radio that Fillon “cannot say he is the candidate of honesty and not be able to answer this”.

Thierry Solère, Fillon’s spokesman, said Penelope Fillon had “indeed” worked for her husband in parliament. He told AFP: “It is common for the spouses of MPs to work with them.”

Bernard Accoyer, a Fillon ally and secretary general of his Les Républicains party and former head of the lower house of the French parliament, told France Inter that Fillon had always worked with his wife. Accoyer said he had often seen Penelope Fillon taking part in her husband’s work. “I saw her at the parliament,” he said. He was vague on details, adding that he didn’t know the exact job she did for her husband.

Citing pay slips, Le Canard Enchaîné claimed that Penelope Fillon, known as Penny, was paid from 1998 to 2002 from funds intended for parliamentary assistants. The Guardian has not seen the pay slips.

From 2002 to 2007, when Fillon took up a cabinet post under then president Jacques Chirac, she became an assistant to Marc Joulaud, who carried out Fillon’s parliamentary duties in his place, earning €6,900-€7,900 a month.

A colleague of Joulaud’s told the paper: “ [I] never worked with [Penelope Fillon]. I have no information about this. I knew her only as a minister’s wife.”

The paper claimed that Penelope Fillon was again paid “for at least six months” in 2012 when Fillon, after serving as prime minister, left government following the defeat of rightwing president Nicolas Sarkozy.

“In total, Penelope will have earned around €500,000 from parliamentary funds,” according to the paper.

Separately, the publication said Penelope Fillon was paid around €5,000 a month between May 2012 and December 2013 by the literary publication Revue des Deux Mondes, which is owned by a friend of Fillon, Marc Ladreit de Lacharrière.

The director of the monthly magazine, Michel Crepu, told France Inter that Penelope Fillon had twice provided work but he had “never ever seen her at the newspaper or bumped into her” at its offices. “I’m not saying she didn’t do the work, but I didn’t see it,” he added, suggesting she provide proof as quickly as possible of the work she did there.

François Fillon told a television interviewer in November last year that his wife stayed at home in Sarthe while he worked as a politician in Paris. “I didn’t have much time to see the first four [of five children] grow up because I was an MP,” he told an M6 TV show about politicians’ family lives. “It was 24/7, so basically they were raised by their mother.”

But he also said, without detailing to which time period he was referring: “She was very involved in the campaigns, handing out flyers and attending meetings with me.”

Fillon told the TV show last autumn that Penelope Fillon was no longer involved in politics at all.

A trained lawyer, Penelope Fillon told the Sunday Telegraph in 2007, after her husband became prime minister, that she preferred being at the couple’s 12th-century chateau near Le Mans, western France, with her children and five horses than in Paris. She said of the city: “I’m just a country peasant, this is not my natural habitat.”